This invention relates to RF load and source pull testing of medium and high power RF transistors and amplifiers using remotely controlled electro-mechanical impedance tuners. Modern design of high power RF amplifiers and mixers, used in various communication systems, requires accurate knowledge of the active device's (microwave transistor's) characteristics. In such circuits, it is insufficient for the transistors, which operate in their highly non-linear regime, close to power saturation, to be described using non-linear numeric models only.
A popular method for testing and characterizing such microwave components (transistors) in the non-linear region of operation is “load pull”. Load pull is a measurement technique employing microwave tuners and other microwave test equipment. The microwave tuners are used in order to manipulate the microwave impedance conditions under which the Device Under Test (DUT, or transistor) is tested (see ref. 1); this document refers hence to “impedance tuners”, in order to make a clear distinction to “tuned receivers (radios)”, popularly called elsewhere also “tuners” because of the included tuning circuits (see ref 2).
Impedance tuners in particular “slide screw tuners” consist, in general, of a transmission line and a number of adjustable parallel tuning elements (22, 30), FIGS. 2 to 5, which, when approaching the center conductor (32) of the airline (31) and moved along the axis of the airline (45) create a variable reactance, allowing thus the synthesis of various impedances (or reflection factors) covering parts or the totality of the Smith chart (the normalized reflection factor area). The relation between reflection factor and impedance is given by GAMMA=|GAMMA|*exp(jΦ)=(Z−Zo)/(Z+Zo), wherein Z is the complex impedance Z=R+jX and Zo is the characteristic impedance. A typical value used for Zo is Zo=50 Ohm (see ref. 3). The equivalent is the Voltage Standing Wave Ratio: VSWR=(1+|GAMMA|)/(1−|GAMMA|).
Up to now such tuning elements are made as metallic probes (slugs) and have had a cubical form (30, 41) with a concave bottom (35) which allows to capture (when closing in to the center conductor) the electric field (99) in FIG. 9, which is concentrated in the area (36) between the center conductor (32, 98) and the ground planes of the airline (31, 97) (FIGS. 3 and 9). This field capturing allows creating high and controllable reflection factors. The disadvantage of this technique is the requirement of very high precision and resolution vertical movement mechanisms (33, 21, 53), which is controlled by a precision stepping motor (20). Because most of the field capturing effect occurs when the probe is very close to the center conductor, a high resolution vertical movement mechanism is needed. This, on the other hand, slows down the tuning procedure, since, when the probe is away from the center conductor, the tuning effect is much less effective, but the vertical moving speed is the same. In praxis, it takes typically 5 seconds to tune from |GAMMA|=0.05 to |GAMMA|=0.1 (VSWR from 1.1:1 to 1.22:1) and only 1 second to tune from |GAMMA|=0.9 to |GAMMA|=0.95 (VSWR from 19:1 to 39:1). Tuning speed is proportional to the change in VSWR divided by the change in vertical position (Y): SPEED=(Δ.VSWR)/(Δ.Y), whereas the vertical position is directly proportional to the motor stepping speed.
The probes used in slide screw tuners cover only a certain frequency range each. The lower frequency limit (61) is due to limited capacitance, the high end (62) due to resonances and propagation cut-off modes. In general the “longer” the probe, the lower the frequency (FIGS. 5 and 6). Hereby “length” of a probe is its size in direction of the axis of the airline. In order to cover a wide frequency range multiple probes are used (52, 55 in FIG. 5); each probe is attached to a precision vertical axis (53) in a mobile carriage (56) and is controlled by a vertical stepper motor (54). In case of even larger bandwidth three parallel probes are used (see ref. 6, FIG. 4a) which requires three axes, triple carriages and three vertical motors.
This invention discloses multiple frequency band probes using a single carriage, without need for a vertical axis and using a single vertical motor.